1. Field of the Invention
The present invention relates to an active matrix substrate for liquid crystal display on which a switching matrix, a plurality of thin film transistors, is formed to drive individual picture elements.
2. Description of the Prior Art
The liquid crystal display having picture elements arranged in a matrix form, as shown in FIG. 23 schematically, is widely used. Such a the liquid crystal display panel is especially widely used for the display of an electronic apparatus, such as a personal computer of lap-top type or the like.
In a further liquid crystal display panels, so called active matrix substrate are used on which a number of thin film transistors are formed in a matrix form. As shown in FIG. 22 schematically, each thin film transistor (TFT) switches on the corresponding transparent electrode PE for a picture element when designated through both the gate and source bus lines, GBL and SBL, respectively.
FIG. 23 shows a cross-sectional view along A--A line of FIG. 22.
An insulation film 1a is formed on a glass substrate 1 and a gate electrode 3 is formed together with the gate bus line GBL on the insulation film 1a by etching Ta film.
This gate electrode 3 and the gate bus line GBL are covered with an anodic oxide film 4 of Ta.sub.2 O.sub.5 and a gate insulator 5 of SiNx is formed so as to cover the whole of the insulation film 2 including the anodic oxide film 4. On the area of the gate insulator 5 covering the anodic oxide film 4, a thin film transistor TFT is formed to switch on a transparent electrode 6, for each of picture elements formed on the gate insulator 5. The thin film transistor (TFT) is formed so as to have a drain electrode 7 connected with the picture element electrode 6, a source electrode 8 connected to the source bus line SBL, and a semiconductor film 9 of amorphous silicon (a-Si) formed above the gate electrode 3. This semiconductor film 9 is connected to the drain electrode 7 and the source electrode 8 through a film 10 of amorphous silicon, and is further covered by a protection film 11.
According to this structure, TFT is switched on by applying a predetermined voltage to the gate electrode 3 through the gate bus line GBL. Therefore, a voltage applied to the source bus line SBL is applied to the picture element electrode 6 through the a-Si semiconductor film 9.
The gate bus line GBL and the source bus line SBL are insulated from each other at their crossing zone by an a-Si (i) / a-Si (n.sup.+) layer 12 and covered by an etching stopping layer 13.
In such a structure of the active matrix substrate, if a gate bus line GBL or a source bus line SBL is broken, all of picture element electrodes aligned along the broken bus line becomes inactive. This causes a line defect of an image to be displayed. Also, if a TFT is broken, the corresponding picture element is made inactive.
Conventionally, various efforts regarding the production process have been made in order to avoid these defects. However, it is impossible to avoid them completely by merely improving the production process.